1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor for wet development, and more particularly, to an electrophotographic photoreceptor having high durability for liquid developer and good image characteristics.
2. Description of the Related Art
In electrophotography, a wet development technique using liquid developer has been known for some time, as disclosed in U.S. Pat. Nos. 2,907,674 and 3,337,340. However, since it is necessary to suppress odors emanating from a paraffinic solvent, a main component of a liquid developer, and to take fire prevention measures, a wet development technique is not widely employed. Instead, a dry development technique using a powdered developer has been generally recognized as a representative technique in electrophotography.
In wet development, since toner particles can have submicron diameters, images having high resolution are more easily obtained. Due to such an advantage, this technique is attracting public attention once again. Conventionally, an inorganic photoreceptor, e.g., amorphous selenium, has been used as an electrophotographic photoreceptor for wet development, and there has been no problems encountered with the use of inorganic photoreceptors. However, substantial problems have been encountered with the use of organic photoreceptors.
An organic photoreceptor generally includes a charge transport layer comprising a solid solution containing a binder resin, such as a polycarbonate resin or an acrylic resin, and a charge transport material, which is a low molecular compound, such as a surface layer. These resins have, a tendency of being penetrated by aliphatic hydrocarbon solvents, and the charge transport material being soluble in the solvent. Liquid developers generally include a dispersion of colorant particles in an aliphatic hydrocarbon solvent. Thus, when the liquid developer contacts an organic photoreceptor, the resin or other photoreceptor components may erode by the solvent, resulting in cracking, lower sensitivity or contamination of the developer by the eluted photoreceptor components. To overcome such problems, research and development of organic photoreceptors having high durability against liquid developers are underway, including the following methods:                (1) Polymerization of charge transport materials to prevent elution;        (2) Formation of a surface protection layer having good developer resistance to prevent a solvent from penetrating into a photosensitive layer; and        (3) Increasing developer resistance of a resin to prevent a solvent from penetrating into a photosensitive layer.        
The prior art related to the first method is described, for example, in U.S. Pat. No. 5,030,532. However, a polymeric charge transport material having high solvent resistance is limited in the kind thereof and general-purpose resins cannot be used, resulting in a considerable increase in the cost of raw materials.
The prior art related to the second method is described, for example, in U.S. Pat. No. 5,368,967. However, according to this technique, the manufacturing process of a photoreceptor is complicated. Also, in order to prevent deterioration of photoreceptor characteristics, a surface protection layer has to be formed as thinly as possible, which may cause lower durability.
The prior art related to the third method is described, for example, in U.S. Pat. No. 5,545,499. However, it is quite difficult to attain solvent resistance of a photoreceptor by using a binder resin having increased developer resistance alone, which is not yet put into practice.